Process for transforming alkali-metal monochromates into bichromates or chromic acid



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GERHARD NICOLAAS VIS, OF PARIS, FRANCE.

PROCESS FOR TRAIISFOBMING ALKALIJYZETAL EEONOCHROMATES INTO IBICHROMATES OR CHROIVIIC ACID.

No Drawing. Application filed. July 10,

T 0 all whom it may concern Be it known that L GERHARD NICOLAAS Vrs, of Paris, France, have invented a new and useful Process for Transforming Alkali-Metal Monochromates into Bichromates or Chromic Acid, which process is fully set forth in the following specification. a a

In my previous application dated 22nd January 1918, No. 213,206, it was shown that by the sole action of carbonic acid upon monochromates the greater portion of such, salts can easily be transformed into bichromates with the simultaneous. production of alkali-metal bicarbonates.

In the, application dated the 18th December 1918, No. 267,349, it was shown that the base employed in the Solvay reaction for the manufacture of carbonate of soda is as applicable to alkali-metal monochromates as to sodium chloride. Together with the bicarbonate of soda there isformed ammonium chromate, this latter being decomposed by lime and the calcium chromate thus obtained is transformed into calcium sulphate and alkali-metal bichromate by double decomposition with alkali-metal bisulphates.

This invention relates to a modification of or improvement in the processes of said ap plications and has for its object a process which obviates the use of ammonia while employing alkali-metal bisulphates the same final result being obtained.

According to this invention the whole or part of the monochromate is transformed into alkali-metal bicarbonate and bichromate with the aid of carbonic acid; all the chromic acid present in form of bichromate of sodium being then precipitated as chromate of calcium with the aid of hydrate of calcium, the calcium chromate thus obtained being transformed into alkali-metal. bichromate and calcium sulphate by the ad dition of alkali-metal bisulphates.

The process is carried out in the followinn manner:

Into a solution of sodium monochromate containing more than 40 parts of such salt to 100 parts of solution (a solution saturated at 45 C. for example) is passed a current of carbonic acid (which may be pure or mixed with inert gases) the temperature being maintained at 30 C. The carbonic acid is caused to act until saturation takes place or until, for example,

1919. Serial No. 309,933.

about one half of the sodium monochromate is transformed into sodium bichromate and sodiumabicarbonate. The duration of the operation may be greatly hastened by not pushing the reaction so far as to obtain the maximum yield of the substance.

The liquid now containing a nearly equimolecular mixture of sodium mono and bichromate is separated from the sodium bicarbonate by known means and the latter salt is subjected. to systematic washing. The filtered liquid to which the first wash waters are added is heated to boiling during which a certain quantity of carbonic acid is evolved; when this evolution has finished there is added as much burnt lime in the form of a paste as there is free chromicacid present in the form of sodium bichromate.

If, for example, the process is started-with 2000 lags. of a solution of sodium chromate saturated at 45 0., there is obtained after separation and washing of the sodium bicorbonate 2300 kgs. of a solution composed in round numbers of 500 kgs. of sodium monochromate, 400 kgs. of sodium bichromate and 1400 kgs. of water.

This solution is brought to boiling pointand when the evolution of carbonic acid has ceased there is added while maintaining the temperature at boiling point 90 kgs. of burnt lime slaked with three times its weight ofwater. About 230 kgs. of anhydrous calcium chromate separates in the form of a heavy sandy product which is very easy to wash as the calcium chromate is nearly insoluble in the hot solution of sodium monochromate. After washing with boiling water, the product thus obtained is dried in a centrifugal hydroextractor and while still wet is mixed with its weight of water and the paste obtained is introduced into a solution consisting of 180 kgs. of sodium bisulphate and 350 kgs. of Water previously heated to C. The double decomposition is facilitated by brisk agitation for several hours, the temperature being maintained at 70 C. The solution of sodium bichromate is then separated from the calcium sulphate and the latter washed until it becomes white. The filtrate and wash waters are concentrated the small quantity of calcium sulphate which has passed into solution gradually separating as the water evaporates; when the specific gravity reaches 1.4 the calcium; sulphate becomes absolutely insoluble and with more.

complete concentration 220 kgs. of so-called molten sodium bichrom'ate are obtained;

The filtrate. and the wash waters of the calcium ch ro'i'nate icon-taming about 0.4 f par ts of calcium chromate for every 100 parts-011' water and also in round numbers 750 kgs.

of sodium chromate aretr'eated' atb'oiling" point with sodium carbonate in order to pres cipitatdthe lime p'resent or with the same object with th'ef carbonic acid after cooling; The solution" of sodium mon'ochromate v freedfrom its lime isconcen'trated to a suit{ able degree-3 again treatedwith carbonic acid and then again subjectetl" to the operations above described.

adding to the calcium chromate a hi- 7 sulphate more acid" NaHSOQ, sodiuinbi chromates are obtained which contain i'ree- 'chroni-ic'" acidfin {tlf'e' quantity required; by

' proceeding inthe"m'anner as-before but r'e- 'la'cing th ejbisulpliate with sulphuric acid commercially 'pure chromic acid is 'obt ained One o'f th'e p-rimar-y advantages of the process askabov'e 1 described resides in recu-' pjeration of; sodium bicarbonate whereby thesodaused'iin treating 'the'chrome ore may be repeatedly used'in furna'cingth e ore. A While sodium acid sulfate is', specifically mentionedfin the claims for converting calciu-m chromate' into sodium bichr'omate, it is to be understood that, sulfuric acid mayl're place. the acid I sulfate in part or in Whole,

de'pending on the contentof chrome acid required in' thebichromate," and its use is m be included ithin-tide scope'of the invention.

- Claims: 1 i

= 1. The p ocess of manufacturing alkali metal bichromate from alkali 'metal monochromate n solution containing calcium,

which consists in precipitatingthe calcium insaid solution at boiling temperature by.

means of an alkali metal carbonate, subjecting-- the lime freed solution n) -th e action o'f carbonicacid, separating the alkali. metal bicarbonate from the resulting solution con taining alkali metal bichromate, treating'the latter solution with slaked lime thereby precipitating calcium'chroma'te', and treating" said calciumf chromate -recipitat-e with a l kali metal bisul'fat'e to produce alkali metal bichrom'ate; i

2': The method consistin'gin subject-h1g1 afsolution of alkali metal monochromate to the actionoffcarbonic acid, 'separatin'gthfe treating "said calcium chrom'atei witli alliali metal bisulfate to produce Y alkali-metal I big chromate.

3'; The-method I concentrated. solution: of sodium monochro mate to the action'ofcarbonic acid, 'sep'arat ing the sodium bicarbonate; precipitatefi'rom the resulting" sodium bichromate solution,

heating the latterto boiling, addingsla'ked lime to produce calciufmlchromate'and-so? din-m monochromate; separating the calcium ch roma'te, and reacting" the same with sodium" acid "sulfate" to form sodium, bichrol 1 v i K175 Intestimony Whereoffl have signed this mate;

specification. I

' GERHARD" NICOLAASVISL consisting in subjecting a 

